The Formation Process And Impact Factors Of HAAs Upon Chlorinating Chlorella Vulgaris

Control the formation of disinfection by-products (DBPs) is one of the focus for the safe technology of the drinking water treatment. It is important to investigate the formation process and impact factors on haloacetic acids (HAAs),a kind of DBPs, upon the chlorination of algae in micro-polluted water. Based on the improved analysis method for HAAs, Chlorella Vulgaris, an unicellular green alga generally detected in surface water, was selected as representative of algae in investigating the formation process and impact factors on HAAs. Furthermore, the corresponding mechanisms of metal ions on the formation of HAAs upon chlorinating Chlorella Vulgaris were studied. The main conclusions were as follows:(1) The reliable and accurate determination of HAAs was modified on the method USEPA 552.3 with improving oscillation time and erivatization time to 0.5 h and 2 h, respectively. It showed that dichloroacetic acid (DCAA) and trichloroacetic acid (TCAA), two predominant HAAs, were detected upon chlorinating Chlorella Vulgaris. At pH 7.0, when the concentration of algal cells were 1.75×105 cell/mL, the yields of DCAA and TCAA were 8.87 and 8.38×10-3μmol, respectively.(2) DCAA and TCAA gradually increased with reaction time and reached steady values untill 12-24 h in chlorination. Meanwhile, the yields of HAAs were enhanced with the increase of pH and chlorine dose. DCAA and TCAA varied from 25.4μg/mg TOC to 39.4μg/mg TOC and from 32.5μg/mg TOC to 45.8μg/mg TOC at 24 h when pH ranging from 6.0 to 9.0. The yield of HAAs could reach a stable value with the increase of chlorine dose. When the addition of chlorine concentration was 10 mg/L (residual chlorine was 3 mg/L), the concentration of DCAA and TCAA were 28.89 and 35.18μg/mg TOC, respectively.(3) Metal ions reduced or enhanced the formation of HAAs upon chlorinating Chlorella Vulgaris. The yields of HAAs could reach a stable value with the increase of metal ions. The results showed that Ca²⁺ and Mg²⁺ reduced the formation of HAAs with increasing pH. At pH 9.0, HAAs decreased 28.4μg/mg TOC with the addition of 150 mg L-1 Ca²⁺, and decreased 17.2μg/mg TOC with the addition of 100 mg/L Mg²⁺, respectively. Fe³⁺, Mn²⁺ and Al³⁺ reduced the yields of DCAA and TCAA and the decline of DCAA and TCAA was more significant in low pH than in high pH. At pH 6.0, HAAs decreased 27.7, 24.0 and 23.0μg/mg TOC with the addition of 0.5 mg/L Fe³⁺, Mn²⁺ and Al³⁺, respectively. It was also found that the addition of Cu²⁺ stimulated the formation of DCAA but reduced that of TCAA, and the decline of TCAA was more significant in high pH than in low pH. At pH 9.0, the formation of TCAA reduced 33.4μg/mg TOC with the addition of 0.5 mg/L Cu²⁺.(4) The results showed that the addition of metal ions have different effects on the partical size distribution of algae in chlorination. At pH 9.0, the addiotn of high concentration of Ca²⁺ and Mg²⁺ enhanced the agglomerate of single cells in 1-10μm; At pH 6.0, the addition of Fe³⁺, Mn²⁺ and Al³⁺ improved the agglomerate of algal cells in 20-300μm; However, the partical size distribution of algal cell peformed the different trend with the addition of Cu²⁺, which significantly increased the volume fraction of single cell in 1-10μm and disappered the agglomerate cells.(5) The results of scanning electron microscope demonstrated that the different forms of Fe³⁺, Mn²⁺ under various pH in aquatic solution affected the formation of HAA. At pH 6.0, the coating material of Fe(OH)3(am) was observed to form on the surface of algal cells with the addition of Fe³⁺ and the coating material of MnO2(am), was also observed to form on the surface of algal cells by the oxidization between Mn²⁺ and ClO-, which could promote the agglomerate of algal cells, reduce the contact of NaClO with algal cells and thus decreased the amount of HAAs formation.